1. Field of the Invention
This invention relates to steam turbines and more particularly to annular diffusers for the exhaust from such turbines. More particularly still, this invention relates to annular flow diffusers located at the exit from condensing steam turbines, such diffusers being defined in most cases by an outer flow guide and, for the most part, either a separate inner flow guide or a bearing cone beginning immediately after the last row of turbine blades and ending at the location of the entrance of the exhaust steam into the main structure of the exhaust hood. The inner or outer flow guides, or both, may be adjustable guide vanes.
2. Description of the Prior Art
In condensing steam turbines used in power generation, steam leaving the last row of turbine blades flows, generally, through an annular outwardly flared passage, known as a diffuser, positioned between the turbine enclosure, or casing, and the exhaust hood proper. Such diffuser is defined by an outwardly flared flow guide extending from the turbine casing, to which it is customarily fastened, for 360 degrees circumferentially about the turbine shaft, and an inner flow guide formed at least in part by the outer surface of the bearing cone or in some cases a separate flow guide, or in case of steam turbines equipped with an adjustable guide vane which at least partially surrounds the bearing cone, mainly by the outer surface of the adjustable guide vane. The steam passes from the diffuser into the body of a collector or "exhaust hood" and subsequently discharges from the exhaust hood into a condenser. The most prevalent type of exhaust hood is one located directly above the condenser, or a "downward-discharging" exhaust hood.
The so-called "diffuser" located between the exit from the last turbine blades and the exhaust hood per se is customarily formed from two annular surfaces which guide the exhausting steam from the turbine itself into the exhaust hood, meanwhile, in well-designed diffusers, because of its increasing cross-sectional areas, diffusing, or decelerating, the exhaust steam passing therethrough. This deceleration causes a decrease in the kinetic energy of the steam plus an increase in pressure, the net effect being that the inlet to the diffuser assumes the lowest pressure of the path from the turbine to the condenser so that the steam exhausts from the last turbine blades into a minimum pressure zone thus increasing the velocity of steam flowing through the blades and increasing the energy available to the turbine to do work.
In a typical arrangement, as indicated above, the upper surface of the bearing cone, or of the adjustable guide vane as the case may be, constitutes most of or the entire inner annular surface of the diffuser and the inner surface of an outer flow guide constitutes the outer annular surface having the overall contour or configuration necessary to direct steam into the exhaust hood. The length of a downward-discharging exhaust hood, measured along the axis of a steam turbine, is limited by the bearing span of the turbine. As a result, steam leaving the last row of blades of a turbine must have its direction changed from mainly horizontal to essentially vertical in a relatively short distance which varies about the circumferential extent of the diffuser, but is relatively short at all points. This places a limit on the length of the diffuser located at the turbine exit, since it is inadvisable to have sharp turns in a diffuser such as might be necessary particularly at the top of the turbine in order to extend the diffuser, because sharp turns are known to cause flow separation with resultant eddies and energy losses. In steam turbines built in the U.S.A. the ratio of the length of the diffuser to its height at inlet is customarily quite small, usually being close to one, and often even smaller. To produce a certain amount of diffusion, turbine designers build diffusers having rather large (inlet-to-exit) area ratios. Such designs are, in general, based on information available from studies of flow in diffusers having uniform and incompressible flow at the inlet.
It is desirable to have a large amount of diffusion, or pressure rise, in a diffuser of a steam turbine, because, for any given condenser pressure, there is then produced a lower pressure at the entrance to the diffuser and thus at the exit from the last row of turbine blades, thus increasing the energy available to the turbine to do work and also improving performance of the last row of blades when condenser pressure is higher than the pressure assumed in design of the turbine, thus increasing turbine efficiency. The amount of diffusion a diffuser can produce, however, is limited by the (longitudinal) pressure gradient, the average overall pressure gradient being the ratio of the pressure rise to the length of the diffuser. Such pressure rise in turn depends on the exit-to-inlet area ratio of the diffuser. If the pressure gradient becomes too large, i.e. the walls of the diffuser diverge to steeply, the steam flow will become separated from the walls of the diffuser and the amount of diffusion can be seriously reduced or even entirely eliminated.
Since a diffuser in a bottom condensing steam turbine is of necessity short relative to its height at its inlet, if it is not to be sharply curved, the amount of diffusion which it can produce is, therefore, correspondingly limited. This is especially so for diffusers in which the flow over a large portion is in predominantly an axial direction, that is, in the direction of the axis of the turbine, which as explained is usually desirable.
The flow entering a diffuser located downstream from a last blade row of a turbine has many wakes in it, these being necessarily formed at the trailing edges of blades by the deceleration of flow passing closely to the blade surfaces. Such wakes can be produced by both fixed blades of the turbine and the rotating blades (as well as shrouds on the blades plus any supporting struts interposed in the flow, or tie- or lacing-wires, if any). It is the wakes from the last rotating blades that exert most influence on the flow in the diffuser, any prior wakes having been largely dispersed into the general flow by such terminal blades. Each moving blade provides a wake, i.e. in the typical large turbine, as many as a hundred or more wakes are present. In this respect the actual flow in a diffuser located downstream of a condensing steam turbine differs from the rather thoroughly studied and relatively well understood diffuser flow in which flow at the inlet to the diffuser is uniform. In steam turbines these wakes are especially thick when the turbine operates at condenser pressures higher than the design condenser pressure because under such conditions the boundary layer flow passing over and from the surfaces of the last turbine blades is either on the verge of separation or is partially separated from the blade surfaces.
3. Description of Related Art
The following prior articles contain discussions or disclosures of phenomena and considerations having a bearing upon the present invention.
1. P. G. HILL, U. W. SCHAUB, Y. SENOO: "Turbulent Wakes in Pressure Gradient," Transactions ASME, Journal of Applied Mechanics, vol. 85, Series E, pp.518-524, December 1963.
2. R. W. FOX, S. J. KLINE: "Flow Regimes in Curved Subsonic Diffusers" Transactions ASME, Journal of Basic Engineering, vol. 84, Series D, pp. 303-316, September 1962.
3. J. R. HENRY, C. C. WOOD, S. W. WILBUR: "Summary of Subsonic-Diffuser Data," NACA RM L56F05, 1956.
4. G. SOVRAN, E. D. KLOMP: "Experimentally Determined Optimum Geometries for Rectilinear Diffusers With Rectangular, Conical or Annular Cross Section," in: Fluid Mechanics of Internal Flow, Elsevier Publishing Company, Amsterdam, Netherlands, 1967, (FIG. 17 on page 291, and Appendix B on pages 311 and 312).
5. J. H. G. HOWARD, A. B. THORNTON-TRUMP, H. J. HENSELER: "Performance and Flow Regimes for Annular Diffusers," ASME Paper 67-WA/FE-21, 1967.
6. M. Ye. DEICH, A. Ye. ZARYANKIN: "Gas Dynamics of Diffusers and Exhaust Ducts of Turbomachines," translated by Foreign Technology Division, Wright-Patterson AFB, Ohio, report No. FTD-MT-24-1450-71. Available from Clearinghouse for Federal and Scientific Information, Springfield, Va., as report No. AD 745470, 1970.
7. "Steam Turbines for Large Power Outputs," von Karman Institute for Fluid Dynamics, Lecture Series 1980-6, Rhode Saint Genese, Belgium, 586 pp., Apr. 21-25, 1980.
8. Y. SENOO, N. KAWAGUCHI, T. KOJIMA, M. NISHI: "Optimum Strut Configuration for Downstream Annular Diffusers With Variable Swirling Inlet Flow," Transactions ASME, Journal of Fluids Engineering, vol. 103, pp.294-298, June 1981.
9. M. F. O'CONNOR, K. E. ROBBINS, J. C. WILLIAMS: "Redesigned 26-inch Last Stage for Improved Turbine Reliability and Efficiency," Paper presented at the ASME/IEEE Joint Power Generation Conference, Sep. 17, 1984, Toronto, Ontario, Canada.
10. J. A. OWCZAREK: "Fundamentals of Gas Dynamics," International Textbook Company, Scranton, Pa., 1964.
11. B. K. SULTANIAN, S. NAGAO, T. SAKAMOTO: "Experimental and Three-Dimensional CFD Investigation in a Gas Turbine Exhaust System", Transactions ASME, Journal of Engineering for Gas Turbines and Power, vol. 121, pp. 364-374, April 1999.
In particular the 1963 Hill et al. article from the Transactions of the ASME describes a study of turbulent wakes in pressure gradients in a two-dimensional diffuser. The study concludes generally that, if a very large pressure gradient is present, a wake may even grow rather than decay, and provides a suggested criterion, i.e. restriction of the pressure gradient of the diffuser, which should be satisfied in order to prevent growth of wakes. The Fox and Kline article discusses flow regimes in a curved diffuser having a rectangular cross section and circular arc center line with uniform inlet flow and presents, in the form of a graph, lines locating the first appreciable stall line as a function of turning angle. The third article by Henry et al. gives a summary of information on performance of diffusers with subsonic uniform flow at the inlet, and the fourth article by Sovran et al. describes an extensive experimental study of such flows. The fifth article by Howard et al. discusses performance and flow regimes for annular diffusers with uniform flow at their inlets. The report by Deich and Zaryankin describes various studies made in the Soviet Union with respect to conical, annular and axi-radial diffusers with uniform inlet flow and of various exhaust hood designs. The seventh report issued by the von Karman Institute discusses, in general, large steam turbines and gives a comparison of the optimal geometry for two-dimensional, conical and straight-core annular diffusers with uniform inlet flow. The eighth article by Senoo et al. describes the only known study on the pressure recovery of three (straight core) annular diffusers without any splitter vanes inside in a model in which tests were run with and without (two or four) struts placed just upstream of diffuser inlet, and therefore with and without wakes at the inlet, with and without swirl in the flow. Attention was directed during the investigation toward finding the best strut configuration and orientation, but not toward the effect of wakes on diffuser performance. The ninth article by O'Connor et al. discusses redesign of the last stages of turbines, while the tenth, a book by the present inventor, deals with equations of gas dynamics. The final, and most recent literature reference known to the present inventor, namely the eleventh article by Sultanian et al. is concerned with an experimental and computational study of flow in a straight core annular diffuser for a gas turbine with struts located inside and about half-way through the diffuser (these struts produced wakes but not at the diffuser inlet because they were located approximately at the middle of the diffuser length) plus guide vanes located a relatively long distance upstream of the diffuser to produce swirl in the flow. The diffuser, which modeled "one of the most complex designs in the existing product line" was provided with turning vanes at the diffuser exit. It had a wall angle of about 8 degrees, and a corresponding two-dimensional straight-wall diffuser angle of about 5 degrees. The attention of the study was focused on a comparison between the experimental results and three-dimensional CFD predictions. The measured total pressure loss in the diffuser was found to be higher than predicted. The flow in the initial length of the diffuser was almost uniform because the guide vanes which produced swirl in the flow were placed a very significant distance upstream of the diffuser inlet with a long annular passage of constant cross-sectional area in-between, and, as has already been stated, the struts were placed within the diffuser and not at the inlet. As a result, the inlet flow to the diffuser was uniform, or nearly so, and this study did not shed any light on the effect of wakes in the inlet flow on the flow in the diffuser.
Estimates made using illustrations of exhaust flow annular diffusers of large steam turbines presented in various publications from the late 1960's until early 1990's have provided the following information: the estimated corresponding two-dimensional straight-wall diffuser angles of turbines made by domestic manufacturers determined at a distance of one half of the diffuser height at inlet along a diffuser mean line fall in the range of 6.5 degrees to 16.5 degrees, with the corresponding rates of diffuser cross-sectional area increase being in the range of from 11% to 30%. For the Siemens/KWU turbines the corresponding numbers are from 4.63 degrees to 8.0 degrees, and 8.1% to 14% for the recent (ca. 1988) units, and 11 degrees and 21% for the older units from 1960's to early 1970's. For a Brown Boveri large steam turbine from the 1970's the corresponding values are 5.5 degrees and 9.6%.
There is no published study which would indicate what, if anything, should be done to compensate for wakes in the exhaust steam entering a diffuser, namely if any limit should be placed on the rate of increase of diffuser cross-sectional area, so that the diffuser and turbine performance could be improved in a flow with wakes in it.
There has been a need, therefore, for a method of design and construction of diffusers for steam exhaust from the low pressure stage in steam turbines, which method and construction will, as a practical matter, allow effective diffusion of the exhaust flow from such steam turbine by taking into account the effect of the wakes, inherent in such exhaust flow as a result of flow around blade surfaces, on the diffusion process in the diffuser.
The present inventor has determined through physical and mathematical modeling and analysis details of which are provided in the attached Appendix that the process of decay of wakes in the flow in a diffuser produces on its own a certain amount of diffusion of the fluid flow, and therefore also a pressure gradient, which adds to that which results from the increase of the diffuser cross-sectional area. In order to maintain the magnitude of the pressure gradient the same as in the case of a flow with uniform velocity at the inlet having an optimal amount of diffusion so as to avoid flow separation from the walls of the diffuser, the rate of increase of the diffuser cross-sectional area must in accordance with the invention be correspondingly smaller.